Marker-Assisted Selection Could Boost Sow Performance

The length of a sow's productive life (SPL) in a breeding herd is impacted by reproductive performance, locomotion and structural soundness.

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The length of a sow's productive life (SPL) in a breeding herd is impacted by reproductive performance, locomotion and structural soundness.

SPL is usually defined as either the number of days that a sow remains in the breeding herd or the number of litters that a sow produces. Length of sow productive life or longevity is evaluated by removal, culling and replacement rates, percentage of gilts in the herd, average parity of females in inventory and average parity at removal.

Because reproductive traits carry low to moderate heritability, marker-assisted selection (MAS) may be an effective tool to reduce the culling rate of sows and improve SPL.

In this study, 119 single nucleotide polymorphisms (SNPs) from 95 genes were examined in a commercial sow population with recorded reproductive traits for six parities. SNPs are changes in a single, specific coding unit of the genetic code.

The SNPs association analyses revealed a number of potentially interesting genes associated with total number born, number born alive and with gestation length in several parities. These associated genes could be considered for marker-assisted selection to improve SPL in commercial sow herds.

According to the PigChamp 2007 benchmarking report (http://www.pigchamp.com/summary_archives.html) in the last 10 years, the average culling rate of breeding females was 49.7% and sow mortality rate averaged 9.2%. The two most prominent reasons for culling are reproductive problems and locomotion disorders. Both appear to affect a higher percentage of sows in early parities.

Reproductive traits are low to moderately heritable and have low repeatability across parities. Traditional phenotypic selection, based on reproduction records, is less effective. Marker-assisted selection (MAS) is one method to improve lowly heritable traits.

The identification of genetic markers significantly associated with high sow longevity would allow breeders to select gilts at early ages — prior to the entry of the herd — that would have the best opportunity for increased sow longevity. Genetic suppliers could use the markers to improve selection methods and possibly “fix” the important genes in the population.

The objective of this research was to identify genetic markers or SNPs associated with sow productive traits. A commercial herd involving 2,066 gilts supplied by Newsham Choice Genetics was included in the project.

Six reproductive traits were recorded: total number born (TNB), number born alive (NBA), stillborn number (SBN), mummy number (MN), gestation length (GL) and non-productive days (NPD). The study included six different parities that were comprised of gradually reduced numbers of sows. DNA was isolated and large-scale genotyping was performed.

Twenty-three genes showed significant associations with at least three reproductive traits. For Parity 1, six genes were significantly associated with both TNB and NBA, while four genes were highly significantly associated with SBN. Two genes were highly significantly associated with MN and NPD, respectively.

In later parities, the six genes had significant association with TNB and NBA. Four genes were significantly associated with GL in several parities.

Researchers also recognized four genes were simultaneously associated with reproductive performance, fatness and locomotion traits, implying that these genes have more than one genetic effect on sow longevity-related traits. The study verified that there are genes causing variation in sow productive life; therefore, the use of marker-assisted selection could improve sow longevity.

The National Pork Board, Newsham Genetics, Hatch funding and the State of Iowa and the College of Agriculture funded this research effort.